In many communications networks there exists a need to synchronise the clock signals maintained in the network elements so that the frequencies and possibly also the phases of the clock signals in the network elements are locked to each other sufficiently well. In some communications networks there additionally exists a need to synchronise the time of day values maintained in the network elements so that the time of day values of the different network elements are identical as precisely as possible. In other words, the time of day values maintained in the different network elements should correspond to a common time to a best possible precision. Such a common time is generally called the universal wall clock time. The network elements mentioned above may be routers or base stations of a mobile network, for instance.
In a solution discussed in Mills, David L., “Network Time Protocol (Version 2) Specification and Implementation”, IETF RFC-1119 (Internet Engineering Task Force, Request For Comments) the network elements send to other network elements synchronisation messages based on which each receiving network element adjusts the clock signal maintained by it. Each sending network element produces its synchronisation messages based on its clock signal. Each receiving network element usually receives synchronisation messages from more than one sending network element. A receiving network element is arranged to choose those of the received synchronisation messages that have been sent by a network element the clock signal of which is, according to certain predetermined criteria, of the best quality among the clock signals of the sending network elements. The receiving network element is arranged to adjust its clock signal on the basis of the synchronisation messages selected. In each network element which sends out synchronisation messages, the clock signal may belong e.g. to one of a plurality of predetermined quality categories, and each receiving network element may be arranged to select for the adjustment of the clock signal the synchronisation messages of the sending network element the clock signal of which belongs to the best available quality category.
A receiving network element may cease to receive synchronisation messages corresponding to a certain sending network element. Such a situation occurs e.g. when a sending network element becomes faulty or otherwise goes out of operation or the communications link between the sending network element and receiving network element becomes faulty or otherwise goes out of operation. If those synchronisation messages, which have been selected by the receiving network element for the clock signal adjustment, cease to arrive, the clock signal adjustment has to be continued based on synchronisation messages received from some other sending network element. Changing the clock signal adjustment so as to be based on synchronisation messages received from some other sending network element will often result in harmful oscillations in the frequency and/or phase of the clock signal adjusted.